1. Field of the Invention
The present invention relates to a liquid discharge head for discharging a liquid droplet of an ink liquid to record on a recording medium, and more particularly to a liquid discharge head for performing ink jet recording.
2. Description of the Related Art
An ink discharge method using an electricity-heat transducing element includes first providing an electric signal to a heat generating element placed in an energy application chamber to heat the heat generating element in a state where ink is supplied into the energy application chamber through an ink flow path to fill the energy application chamber. Thus, ink around the heat generating element in the energy application chamber is instantaneously heated and reaches the boiling point and boils, and an air bubble is generated on the heat generating element. A large blowing pressure of the air bubble generated at this time provides kinetic energy to the ink in the energy application chamber, and discharges the ink to the outside through a discharge port communicating with the energy application chamber.
When the ink is discharged by this discharge method, the air bubble generated on the heat generating element grows, and heat of the ink on and around the heat generating element is diffused around after the ink is discharged, thereby reducing a volume of the air bubble. When the air bubble vanishes, the air bubble is crushed and broken by the ink in the energy application chamber. At this time, the air bubble is broken, which may damage a member around the air bubble. Specifically, cavitation caused by driving of the heat generating element may damage a surface of the heat generating element. This damage may reduce recording image quality.
For a recording head disclosed in Japanese Patent Application Laid-Open No. H04-10940, an ink discharge method is proposed in which an air bubble generated on a heat generating element communicates with air when the air bubble grows and ink is discharged. With this ink discharge method, the air bubble communicates with air and thus pressure in the air bubble is reduced to the same level as the air, and the air bubble is not crushed by the ink. Ink corresponding to the discharged ink again fills the energy application chamber. Thus, the air bubble hardly remains in the energy application chamber, thereby preventing occurrence of cavitation and preventing damage to the heat generating element.
As disclosed in Japanese Patent Application Laid-Open Nos. H11-188870 and H04-10940, a discharge method is proposed in which an air bubble communicates with air, specifically, the air bubble once grows to a maximum volume while discharging ink, and then first communicates with air in a volume reduction process of the air bubble. With this discharge method, occurrence of cavitation is prevented. Also, a liquid level in a discharge port after discharge of the ink is lowered in a direction opposite to a discharge direction. Thus, ink to be a satellite droplet is separated from a discharged main droplet, and easily absorbed by a liquid level in an opening of the discharge port. This prevents generation of mist, and allows recording with high image quality.
Further, Japanese Patent Application Laid-Open Nos. 2002-321369 and 2008-238401 propose a method in which a heat generating element is placed offset from a central line of an ink flow path, and a method in which a discharge port is displaced from a center of a heat generating element forward or rearward in an ink supply direction to reduce occurrence of cavitation. This increases durability of the heat generating element.
As such, in the above-described recording head, a liquid discharge method of air communication type is used to prevent occurrence of cavitation. However, with such a liquid discharge method, occurrence of cavitation cannot be completely prevented, but cavitation may occur.
Occurrence of cavitation will be described below with reference to FIGS. 1A to 1E. FIGS. 1A to 1E are sectional views of an ink jet nozzle, and illustrate a shape change process of an air bubble generated on a heat generating element and a liquid level of a liquid to be discharged. FIGS. 2A and 2B are plan perspective views seen in a direction perpendicular to a main surface of the heat generating element.
When ink is discharged, an air bubble 21 once reaches a maximum volume, and then starts to vanish (see FIG. 1A to 1C). Substantially at the same time, a meniscus 22 is formed in a discharge port, and moved toward a heat generating element 23 as the ink is discharged and an amount of ink in an energy application chamber is reduced.
In this movement, ink 24 and the air bubble 21 between the meniscus 22 moved toward the heat generating element and the heat generating element 23 are pressed and compressed (see FIG. 1D). Thus, the air bubble 21 is compressed substantially at a center of the heat generating element 23, and an area of the air bubble 21 facing the center of the heat generating element 23 is recessed to be an annular air bubble 21 as illustrated in FIG. 2A. This phenomenon is more noticeable with a lower height (OH) from a bottom surface of the ink flow path on which the heat generating element is formed to a top surface of an orifice plate having a discharge port. When a clearance between the heat generating element 23 and an energy application chamber wall 26 adjacent to a peripheral end of the heat generating element 23 is small, the annular air bubble 21 collides with the energy application chamber wall 26 and is separated. Also in the air communication type, the air bubble is separated at a collision area before communicating with air (see FIGS. 1E and 2B). This is because the air bubble 21 having become the annular air bubble has high surface tension due to the collision with the energy application chamber wall, and cannot maintain the annular shape.
Among the separated air bubbles 21A and 21B (see FIGS. 1E and 2B), the air bubble 21A on an ink supply side having a large volume communicates with air and has therein pressure of the same level as the air. On the other hand, the air bubble 21B does not communicate with air, and thus causes cavitation in vanishing. The cavitation reduces durability of the heat generating element.
Thus, there is a need to prevent collision of the air bubble with the energy application chamber wall when the air bubble generated by heating of the heat generating element vanishes. Specifically, there is a need to provide a clearance to prevent the collision between an end of the heat generating element and a side wall surface of the energy application chamber adjacent to the heat generating element. This means increasing a distance between the heat generating element and the energy application chamber wall. However, when the energy application chamber is increased in size, energy used for discharge is reduced to reduce a discharge speed of the ink. Specifically, it has been found that ink discharge efficiency is reduced.